This invention relates to a process for preparing, with ease at low cost, an .alpha.-bromo,.omega.-chloroalkane which is useful as an organic synthesis raw material, agricultural chemical or intermediate for pharmaceutical.
In organic synthesis, an .alpha.-bromo,.omega.-chloroalkane is a useful compound indispensable for extending a carbon chain. This compound features that even after a substituent is selectively attached only to one of bromo (--Br) side and chloro (--Cl) side by making use of the difference in the reactivity between them, a target compound can be synthesized using a halogen group on the other side. For example, it is possible to selectively react an organic nucleophilic reagent such as Grignard reagent only with the bromo side of an .alpha.-bromo,.omega.-chloroalkane.
What is important here is a purity of an .alpha.-bromo,.omega.-chloroalkane obtained by the reaction. When it contains an .alpha.,.omega.-dichloroalkane as an impurity, it is, in some cases, inert to the above-described nucleophilic reagent. When it contains an .alpha.,.omega.-dibromoalkane, reaction occurs at both the .alpha. position and .omega.-position of the compound, which inevitably forms new impurities. Accordingly, it is very important to find a method to heighten the purity of an .alpha.-bromo,(.omega.-chloroalkane.
As a conventional method, it is the common practice to prepare an .alpha.-bromo,.omega.-chloroalkane, for example, by chlorinating one (.omega.-position) of the hydroxyl groups of the corresponding .alpha., .omega.-dihydroxyalkane with hydrochloric acid or the like and then brominating the remaining hydroxyl group at an .alpha.-position with phosphorus(III) bromide or the like as shown by the following reaction scheme: ##STR1##
The above conventional technique is however accompanied with the problem that since it needs selective halogenation of the hydroxyl group only at one side, the yield of an .alpha.-bromo,.omega.-chloroalkane inevitably becomes low relative to the raw material .alpha.,.omega.-dihydroxyalkane. Moreover, even if selective halogenation can be accomplished, disproportionation of the resulting halogen occurs easily without close attention upon distillation and purification, which results in the problem of byproduction of an .alpha., .omega.-dichloroalkane and .alpha., .omega.-dibromoalkane as impurities. According to the conventional technique, the scale-up therefore cannot be actualized easily and furthermore, it leads to a cost increase.
An object of the present invention is to overcome the above-described drawbacks of the conventional technique and to provide a process for the preparation of a useful .alpha.-bromo,.omega.-chloroalkane easily at low cost.